Intaglio printing is a secular printing technique which consists of printing patterns in relief by means of an engraved printing plate (which is designated within the context of the present invention by the expression “intaglio printing plate”). The intaglio printing plate is inked on its surface by means of one or several inks. The surplus of ink outside the engravings is then wiped and pushed inside the engravings. The thus inked and thus wiped printing plate is applied against a printable substrate (for example security paper) in a press exerting a high printing pressure and pressing the substrate against the printing plate in such a manner that the ink from the engravings is transferred onto the substrate. The result is a document comprising a printed pattern reflecting the engraved patterns with an additional relief or embossing, matching the printed pattern, which relief or embossing reflects the depth of the engravings and which is typically recognizable to the touch.
Intaglio printing presses, such as used for producing paper securities, in particular banknotes, are for example described in Swiss patent No. CH 477 293 A5, in European patent applications published under Nos. EP 0 091 709 A1, EP 0 406 157 A1, EP 0 415 881 A2, EP 0 563 007 A1, EP 0 873 866 A1, EP 1 602 483 A1, and in International applications published under Nos. WO 01/54904 A1, WO 03/047862 A1, WO 2004/026580 A1, WO 2005/118294 A1, WO 2011/077348 A1, WO 2011/077350 A1, WO 2011/077351 A1, all in the name of the present Applicant.
For a long time, intaglio printing plates were engraved by hand in a plate of soft metal, for example copper, brass or another suitable metal or metal alloy. The tools used by the engraver were typically constituted of burins or styluses the end of which was sharpened and suited to the desired dimensions of the patterns to be engraved, the latter patterns being constituted essentially of lines and curves modulated in dimensions, according to the action exerted by the engraver, to produce variations in tone reproducing the half-tones of the image to be engraved, for example a portrait.
More recently photolithographic techniques have been proposed in order to facilitate the transfer of images on the intaglio printing plates, as well as computer-assisted engraving processes.
The intaglio printing technique is especially used in the field of printing of paper securities, in particular for printing banknotes, intaglio printing remaining one of the most difficult printing techniques to counterfeit.
In the context of printing of paper securities, in particular printing of banknotes, techniques have been developed for assisting the engraver in his task, in particular with a view to reducing the required engraving time as well as the time for producing printing plates. The approach adopted up until very recently consisted in producing a unique engraved original (by hand or assisted by technical engraving means) representing one single document to be printed, and replicating this original as many times as needed to produce a printing plate comprising several replicas identical to said original. According to this approach, the employed engraving technique adheres essentially to that of an engraver, that is to say, that each pattern is engraved according to the outline of the relevant line or curve, that is to say in a substantially vectorial manner (see for example International application published under No. WO 97/48555 A1).
The present Applicant has proposed an innovative approach for producing intaglio printing plates, an approach which is described in International application published under No. WO 03/103962 A1 (which application is incorporated by reference in its entirety within the context of the present application). This process consists in generating a set of three-dimensional digital data constituted of pixels each representing an elementary point to be engraved in the surface of the plate, the engraving being operated pixel by pixel on the basis of said three-dimensional digital data. According to this technique, a printing plate may be engraved directly. Alternatively, a printing plate precursor (advantageously a metal plate having a polymer layer) may be engraved. In this last case, it is the polymer layer that is engraved and the thus engraved precursor is then used to make printing plates by galvanic replication. Furthermore, the engraving is advantageously performed by laser. This engraving process, whether direct or indirect, is put into practice commercially by the Applicant under brand name CTiP® (Computer to Intaglio Plate®) and has become so to speak a standard in the security printing industry.
Contrary to the prior approach which consisted in making a unique engraved original representing one single document to be printed, a plate in its entirety may be engraved in one single phase, without having to undergo the tedious process of replicating an original. According to the technique described in International application No. WO 03/103962 A1, the replication of the original on the plate is carried out within a digital environment, thus allowing in particular to compensate for the distortions of the paper occurring during intaglio printing, a compensation which was simply impossible by using the previously employed techniques. Furthermore, the technique described in International application No. WO 03/103962 A1 allows creating patterns which extend from one banknote to another, without discontinuities, that is to say patterns which extend up to the edge of the banknote without interruption (as can be seen on the illustration of FIG. 1 discussed further below).
A considerable advantage of the aforementioned technique resides in the fact that it is substantially independent from the complexity of the patterns to be engraved, whereas the prior techniques are dependent on the level of complexity of the engravings to be performed.
An evolution of the technique described in International application No. WO 03/103962 A1 for the direct engraving of intaglio printing plates, is described in International application published under No. WO 2009/138901 A1 (which application is also incorporated by reference in its entirety within the context of the present application).
These various computer-assisted techniques have allowed the development of a large variety of security elements taking advantage of the advantageous properties of intaglio printing, amongst which in particular dual-tone or multi-tone elements which make use of the variations in tonal value of the intaglio ink, continuous background, latent images, positive/negative micro-print, tactile elements, etc. Thanks to these techniques, security elements of high complexity, allowing to effectively fight counterfeiting, may be created. Illustrating examples are presented in particular in International applications published under Nos. WO 2005/090090 A1 and WO 2007/119203 A1.
FIG. 1 shows by way of illustration a reproduction in black and white of a banknote specimen produced by the Applicant and distributed to the public on the occasion of the XIXth International Security Printers Conference organized by Intergraf (www.intergraf.eu) and which was held during the month of May 2003 in Montreux (Switzerland). This specimen, representing Lord Byron, was produced according to the aforementioned CTiP® process, the set of elements visible on FIG. 1 constituted of multicolored elements printed by intaglio printing. One in particular recognizes multi-tone elements around and underneath the portrait of Lord Byron (on the right) as well as around and under inscriptions “CTIP” and “COMPUTER TO INTAGLIO PLATE” (on the left), as well as a set of other intaglio elements taking advantage of the possibilities offered by the CTiP® technology. This specimen illustrates the degree of complexity of elements which may be created by intaglio printing, particularly by means of the aforementioned CTiP® process.
A difficulty resulting from the availability of the modern techniques allowing to create intaglio elements resides in the fact that the engraver (one may also talk of “designer” in as far as the intaglio security pattern is as of now created in a computer environment) has a near unlimited freedom with respect in particular to the definition of dimensions (line width, depth, etc.) and profiles (square, “U-shaped”, “V-shaped”, etc.) of engravings. Nevertheless, this freedom is not directly transposable to the print, namely that not any type of engraving is necessarily printable. Intaglio printing remains subjected to physical and mechanical constraints of which it is not possible to be free of. By way of extreme illustration, it is theoretically possible to design and create engravings of fine width and great depth, for example an engraving exhibiting a line width of 10 μm for a depth of 100 μm, however, such a structure can hardly be inked and cannot be correctly printed in as far as the substrate cannot deform in order to “get” the ink inside such an engraving. Likewise, an engraving of large surface area (several mm2) is not printable without there being provided adequate structures for retaining the ink inside the engraving. Indeed, in the absence of such ink retaining structures, wiping of the intaglio printing plate would lead to an overly important removal of ink applied inside the engraving. Compromises must therefore be made in practice, these compromises particularly implying a certain correlation between the line width, the depth of the engraving, and the profile of the engraving. These compromises must also take into account the overall security pattern to be intaglio printed, as even though one individual element is potentially printable with certain printing parameters, these printing parameters could well not be adapted for printing the other elements of the security pattern. The quality of an intaglio print is thus directly linked to the quality of realization of the plate. It is also worth mentioning that other factors influence the quality of an intaglio print, namely, in particular:                the quality and complexity of the substrate to be printed, particularly the integration of more and more varied and numerous security elements;        the intaglio inks which typically exhibit variations in viscosity, different formulations, various drying properties, etc.;        machine settings, particularly printing pressure, inking, contact settings, temperature, etc.        
Regarding machine settings, the printer must in particular proceed with a setting of the printing pressure (that is to say, the pressure exerted between the intaglio printing plate and the substrate to be printed), the ink coverage (that is to say, the quantity of ink applied onto the intaglio printing plate) and the wiping. These printing parameters notably influence the intaglio printing result. Mastering these various printing parameters is therefore crucial, particularly in order to ensure a good repeatability of the print and avoid overly important variations in terms of printing quality.
In practice, the printer is therefore confronted essentially with two main issues, namely:
(i) to ensure that the set of engravings forming the security pattern to be intaglio printed be indeed printable (it can thus be spoken of a verification of the printability of the security pattern and a validation of the engravings); and
(ii) to further ensure that the set of intaglio elements forming the security pattern may be printed with regularity and constant quality, and this with printing parameters that are most suitable for mass production (it can thus be spoken of a verification of the repeatability and the variability of the print).
This however requires that the printer may be in a position to objectively evaluate and measure these characteristics on the final result as printed.
A purpose of the present invention is to meet these needs.